The invention generally relates to conductive and non-conductive materials that are used in conjunction with providing an electric field at one side of such a material responsive to an electric field on the other side of the material for biomedical applications.
The design of an electrically conductive pressure sensitive adhesive (PSA) for biomedical applications has long presented challenges at least because adhesive strength and flexibility generally decrease with increased electrical conductivity. The materials that are typically used (e.g., added) to provide good electrical conductivity are generally less flexible and inhibit adhesion. A conventional way to prepare a conductive coating is to fill a polymeric material with conductive particles, e.g., graphite, silver, copper, etc., then coat, dry and cure the polymeric binder. In these cases, the conductive particles are in such a concentration that there is a conductive network formed when the particles are each in physical contact with at least one other neighboring particle. In this way, a conductive path is provided through the composite.
For pressure sensitive adhesives (PSAs), however, if the particle concentration is high enough to form a network in which particle-to-particle contact is maintained then there is little chance that the polymer (e.g., elastomer) system of the PSA component is present in high enough concentrations to flow out to make surface-to-surface contact between the substrates and an electrode, i.e., act as an adhesive. Conversely, if the PSA component is in sufficient concentration to make sufficient surface contact to the substrate, it would have to interrupt adjacent conductive particles such that particle-to-particle contact is disrupted.
Another type of electrically conductive PSA includes conductive spherical particles with diameters equal to or greater than the thickness of the PSA. In this fashion the signal or current may be carried along the surface of the particles, thus providing current flow anisotropically in the z dimension of the adhesive. Such a composite has not been shown in the prior art to be usable for a biomedical adhesive.
Salts, such as sodium or potassium chloride, readily dissolve when in an aqueous medium, and their ions dissociate (separate into positive and negative ions). The dissociated ions may then convey an electrical current or signal. For this reason, salts have long been added to water, which then may be added to polymeric and elastomeric materials, to provide good electrical conductivity. For example, U.S. Pat. No. 6,121,508 discloses a pressure sensitive adhesive hydrogel for use in a biomedical electrode. The gel material is disclosed to include at least water, potassium chloride and polyethylene glycol, and is disclosed to be electrically conductive. U.S. Pat. No. 5,800,685 also discloses an electrically conductive adhesive hydrogel that includes water, salt, an initiator or catalyst and a cross linking agent. The use of such hydrogels however, also generally requires the use of a conductive surface at one side of the hydrogel (away from the patient) that is capable of receiving the ionicly conductive charge, such as silver/silver chloride, which is relatively expensive.
While these hydrogel/adhesives can have good electrically conductive properties, they often have only fair adhesion properties. Another downside is that the electrical conductivity changes with changing water content, such as changes caused by evaporation, requiring that the hydrogels be maintained in a sealed environment prior to use, and then used for a limited period of time due to evaporation.
U.S. Pat. No. 7,651,638 discloses a water insensitive alternating current responsive composite that includes a polymeric material and a polar material (such as a salt) that is substantially dispersed within the polymeric material. The polar material however, is not employed to provide electrical conductivity via ionic conduction. The polymeric material and the polar material are chosen such that the two materials each exhibit a mutual attraction that is substantially the same as the attraction within the individual materials. Because of this, the polar material neither clumps together nor blooms to a surface of the polymeric material, but remains suspended within the polymeric material. This is in contrast to the use of these salts in other applications that are intended to bloom to a surface to provide a conductive layer along a surface, e.g., for static discharge.
The composites of U.S. Pat. No. 7,651,638, however, remain dielectrics and have high resistance, and are therefore not suitable for use in certain applications, such as providing electrical stimulus to a human subject (defibrillation and/or transcutaneous electrical nerve stimulations, etc.) due to the high resistance of the material. This type of signal detecting adhesive is also not capable of dissipating the charge overload in a timely enough fashion as per AAMI EC12-2000-4.2.2.4, which is directed to defibrillation overload recovery (DOR). The materials are therefore not suitable for use as a monitoring electrode through which a signal may be needed to be detected after a defibrillation charge is applied to a patient. The failure to pass AAMI EC12-2000-4.2.2.4 is due to the high impedance of these capacitively coupled adhesives.
There remains a need, therefore, for a composite for use in conducting a representative signal and/or current through at least the z dimension of a PSA in a biomedical electrode, such that the use of conductive particles may be minimized, while preserving the adhesive's properties, so that both good electrical performance and good adhesive properties may be maintained.